SZ 6 is a terrible spot for the Japanese navy if you’re being focused by the WAllies (it’s fine if you’re left relatively alone and are threatening Hawaii for victory). The DEIs are worth 20 to Japan, Japan+Korea money being convoyed is only 11. The Philippines are the best spot to station the IJN after conquering the DEIs. Add an airbase to FIC and you’re basically unapproachable. Not only does it extend that air’s reach, it also provides a scramble to protect newly built transports/destroyers to reclaim DEIs from the WAllies/reinforce IJN
The US is going to need a massive fleet to be able to survive in SZ 6/Carolines SZ or use several destroyers as blockers. Every plane you have in FIC w/ an airbase can hit SZ 6, same with the whole fleet at Philippines if it’s not blocked. Carolines SZ isn’t much safer because if you still control one of the islands in the adjacent sea zones (not uncommon as they are valueless, defended, and no facilities), you can “double up” your aircraft carrier capacity when attacking it: Planes on the carriers at Philippines attack the sea zone planning to land on said island and planes from FIC can attack the sea zone planning to land on the carriers. If they do block you with destroyers, then you can kill those with subs and/or planes and take the favorable casualty ratio. Since the US needs a heavily defensive fleet to be able to withstand your counterattack, your fleet at the Philippines is likely unattackable by the US fleet, putting planes on the island for scramble if need be.
If Russia stacks Bury Rd1, just don’t do a J1 DoW. Crush the Russian stack J1 instead, advance on China and set up a moderately successful J3 DoW (not gonna be ideal because of the Russian diversion. But hey, you get Russian money as a trade-off). Be content that Germany can get a comfortable take of Moscow without worrying about balancing haste and profitability.
If Russia pulls back for one round and then re-advances to Bury Rd2, this is probably the toughest Japan will have it. Very unlikely that they will take the Pacific victory (unless the US ignores them), but Germany will still get the easy take on Moscow and should be able to turn that into a Europe victory.